Not an odd question at all.
Marty is right: Sunbrella brand canvas fabric is best suited for this purpose, the heavier the better, up to a point.
This reputable vendor sells all different weights of Sunbrella fabric:
Sailrite.com | Home
There are also a few places you can buy a pre-made sail cover. I've never tried this though.
I guess you have a sewing machine and sewing experience?
Hand sewing Sunbrella should be easier than heavy dacron sail cloth.